1. Field of the Invention
The invention relates to a fuel injection timing control device for an internal combustion engine.
2. Description of the Related Art
Japanese Unexamined patent Publication No. 61-2050 discloses a fuel injection timing control device which starts and ends the fuel injection before the intake valve opens, i.e., before the intake stroke, when the engine has warmed up, and which starts and ends the fuel injection after the intake valve opens, i.e., during the intake stroke, when the engine has not warmed up. Accordingly, during fuel injection when the engine has warmed up, all of the injected fuel impinges onto the inside wall of the intake port so that some of the fuel rebounds therefrom as small droplets. Further, the fuel that adheres to the inside wall vaporizes due to the heat of the wall. As a result, during the intake stroke, the small droplets and the vaporized fuel are mixed adequately with the intake air and thus a good air-fuel mixture can be supplied to the combustion chamber. On the other hand, during fuel injection when the engine has not yet warmed up, the injected fuel is atomized in the intake air and is supplied to the combustion chamber with the intake air. In that manner, most of the fuel injected can be supplied to the combustion chamber without sticking to the inside wall of the intake port. Initially, if the injected fuel sticks to the inside wall of the intake port, the fuel cannot be vaporized because the inside wall is still cold prior to the engine warming up. Thus, the fuel flows into the combustion chamber as liquid fuel so that proper combustion deteriorates.
During fuel injection which starts and ends before the intake stroke, as discussed above, some of the injected fuel sticks to the inside wall of the intake port. Although some of fuel on the inside wall is vaporized during the intake stroke and is supplied to the combustion chamber, the remainder of the fuel that originally stuck to the inside wall remains on the intake port and is not supplied to the combustion chamber. However, some of the fuel which remains on the inside wall of the intake port from the prior fuel injections does vaporize and compensates for the current fuel that does not vaporize so that the required amount of fuel is supplied to the combustion chamber.
Accordingly, the fuel injection which starts and ends before the intake stroke needs a certain amount of fuel stuck or adhered to on the inside wall of the intake port. However, in the above fuel injection timing control device, when the fuel injection starting and ending are changed directly from during the intake stroke to before the intake stroke, that is, going from cold start operation to warm engine operation, the amount of fuel which is stuck or adhered to the inside wall of the intake port is small because the fuel injection has started and ended during the intake stroke so there is little previously adhered fuel to compensate for the currently injected fuel stuck to the intake port that does not vaporize. Thus, the air-fuel ratio of the mixture temporarily becomes very lean and the drivability of the vehicle deteriorates during that period.